1. Field of the Invention
This invention relates to a method and an apparatus for image processing that is applicable to devices such as a copying machine and an image scanner.
2. Related Background Art
Copying machines have been improved to reproduce a color image of higher quality as close as an original one. Such improvement results in an increased chance for the copying machine of being abused to counterfeit banknotes or valuable securities. With respect to this, various systems have been developed that are capable of discriminating banknotes or the like.
For example, U.S. patent application Ser. No. 715,922 discloses a system for recognizing line drawing of a pattern on the original image. Another system is disclosed in U.S. Pat. No. 5,216,724 where color components are recognized on the original image.
In addition, U.S. patent application Ser. No. 139,173 disclosed to print a particular stamp on a banknote itself with an ink containing fluorescent dye, allowing discrimination between the genuine and counterfeit banknotes. The Application also suggests to use other types of ink such as the one having an infrared absorption characteristic.
A device designed to detect such infrared comprises a reading sensor for infrared detection along with a reading sensor that is similar to those provided in typical devices for reproducing color images. The device of the type described is capable of detecting specific information involved in the original image according to the information about visible color components and luminous energy of the infrared.
When the infrared is used as a basis for discriminating a specific stamp, an erroneous detection may be made due to the presence of some infrared absorbing materials on the back of the original. More specifically, the infrared reaches the deep inside of a paper as well as being reflected from the face of the paper because it involves wavelengths longer than those of visible light. A portion of the infrared reached to the deep inside is reflected therefrom in an amount depending on the infrared absorbing materials, if any, present there. Carbon black, which is often contained in inks and pigments for printing, is one of the infrared absorbing materials. Thus, if the carbon black is present on the back of the original, the infrared reached to there is absorbed by this carbon black. However, less or no visible information is obtained from the back of the original. Thus, the visible information is obtained from the face of the original though the infrared information is obtained from the back of it. As a result, there is a disadvantage of erroneously detecting the infrared absorbing materials on the back of the original as the specific stamp.
Alternatively, the ultraviolet light may also be used for detecting a specific stamp on the original. A fluorescent dye contained in the ink is, however, more likely to be detected as the specific stamp causing incorrect discrimination of the original.
In addition, the device for reading the color image (e.g., a color copying machine and an image reader) should be modified to some degree if the invisible pattern is used as an identifier for a specific original. This means that the device should be capable of focusing visible regions as well as invisible regions, which is difficult for the existing devices.
Another challenge to the security is to use a glazing color having specific reflecting characteristics to the infrared region and thereby detecting the latter. The glazing color can be printed independently of the visible colors, so that this technique is applicable to make any valuable securities. However, this optical characteristics specific to the infrared region is disadvantageous in that it is affected by various factors including the color material such as the carbon black, oil and dirt of fingers. Thus, incorrect discrimination may sometimes be caused.